Onslow was educated at Exeter College, Oxford (BA 1846).
Taking Holy Orders in 1847, he was for some time curate of Longdon and of March. In 1859, he became rector of Upper Sapey, Herefordshire.
His hymns, mainly translations from the Latin, appeared in Lyra Messianica, Lyra Mystica and Lyra Eucharistica. He also wrote some prose works, and contributed articles to the Dictionary of Christian Antiquities.
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